As will be appreciated from reference to U.S. Pats. No. 4,602,227 and 4,723,307, it is very desirable, particularly in television broadcasting, that the broadcaster be able to readily choose to couple the maximum possible television signal power to the antenna and to keep the broadcasting station operating under all conditions. One preferred way of doing this is to use two transmitters or high-power amplifiers in parallel and to couple the power therefrom to the antenna. Thus the advantage of improved reliability is obtained in that operation at reduced power continues if one of the transmitters fails. There is the further advantage that a given output power can be achieved by the use of a plurality of inexpensive low-power output stages rather than by means of a single expensive high-power unit.
So-called switchless switching arrangements have been developed in order to allow switching of a transmitter output while energized, and this usually takes the form of phase shift controller switching. Generally speaking, an arrangement for such non-contact switching includes a directional or hybrid coupler for combining the outputs of two transmitters and for coupling them by way of two legs or paths to a further hybrid coupler having one of its outputs coupled to the antenna and another coupled to a dummy or waste load. Each of the two legs or paths includes a controllable phase shifter. Such an arrangement allows both transmitters to be operated simultaneously, and allows the signals to be switched between the antenna and the waste load without the switching of contacts and without de-energizing the transmitters. Instead, the switching is often accomplished by selective control of the reactive terminations associated with the circuit. Control of these reactances causes the signals arriving at the antenna or at the waste load by the two paths to be either in-phase and therefore add, or to be out of phase and therefore cancel. One form of controllable phase shifter involves a coaxial variable conductance-capacitance circuit, such circuit including a series coaxial capacitance formed by a hollow inner conductor having a gap which is centered in a coaxial inner conductor. An insulated conductor slug is located within the hollow center conductor and is movable between a position straddling the gap and a position remote from the gap by varying the capacitance across the gap. A series inductance trims the capacitance. This arrangement of the controllable phase shifter is advantageous, as may be appreciated by reference to the aforenoted U.S. Pat. No. 4,723,307.
However, the approach taken to non-contact switching of signals in that patent seeks to avoid some of the disadvantages associated with the earlier known coaxial variable inductance-capacitance scheme; which scheme does present certain packaging problems because four hybrid or directional couplers and two independently actuated sets of reactive terminations are required, each set including a pair of simultaneously actuated coaxial inductance-capacitance circuits.
Accordingly, in order to overcome these drawbacks the approach in U.S. Pat. No. 4,723,307 is to provide non-contact switching among sources and loads by a simplified apparatus as described therein. However, this approach has certain limitations of its own when it comes to variability of input power ratios, that is, differences in the two power levels involved in the first and second sources or transmitters. In the switching systems described in U.S. Pat. No. 4,723,307 the phase shifters are limited to selecting three possible phases such as 0, -90, and -180 degrees. This is fine in those situations where the power from the A and B sources are equal, let us say, each having a value of 100 kilowatts, or where one of the sources has a value of 100 and the other a value of 0.
Specific reference to FIG. 1 herein illustrates such a switching system as is described in U.S. Pat. No. 4,723,307. It will be noted that two sources, A, B, each at zero degrees, are fed to a first, -3dB, 90 degree hybrid coupler 10, are combined therein and the combinations are sent by separate output ports 12 and 14 to phase shifters C and D in the respective coaxial paths or legs. A further hybrid coupler 16, shown in the specific form of a Magic Tee, is provided with both a normal or main output and a waste load output as indicated.
As will be understood by those skilled in the art, the first coupler, that is, coupler 10, produces at one of its output ports, for example port 12, the sum of the first signal, or signal A, plus a relatively phase-shifted (90 degrees) second signal, or signal B; whereas at the second output port, i.e., port 14, the opposite of the first situation obtains; that is, the sum of the second signal plus a relatively phase-shifted first signal. Likewise, for the second coupler in the form of a Magic Tee 16. The effects of the phase shifting arrangement, in the form of phase shifters C and D in each of the respective legs 18 and 20, will be apparent from the table appearing. immediately below the diagram in FIG. 1.
In the past, if one wanted to build a switchless RF switcher in either coaxial construction or in wave guide, the coupler was used with a coupling value equal to the ratio of the lowest input power compared to the combined output power. Thus, two equal inputs produce an output equal to twice either of the inputs. Accordingly, a -3 dB coupler would be employed. However, if one wanted to combine two inputs of unequal power, for example, where one input level is equal to half of the other input level, the ratio of the lowest input to the output is then 1:3 or -4.77dB.
In each of the above cases, that is in either the -3dB coupler case or the -4.77dB coupler case, the output of the combination could be maintained to approach 100% of total input by proper phasing of delay lines to a second coupler which is adjusted to -3dB. Reference to FIG. 1 will make this quite clear; that is, the use of couplers and phase delays to achieve 100% combining. In the illustrated case in FIG. 1, the phase control provides the ability to combine both transmitters or sources A or B individually (where, for example, both have an input power level of 100) into the normal output or the waste load for test purposes. The phase control, of course, also provides the ability to feed either transmitter into either the normal output or the waste load when the other transmitter output power level is 0.
However, a serious problem is presented when the two inputs are unequal in their primary design mode of operation such as, for example, if the A input has a value of 100 and the B a value of 50.
Accordingly, it is a primary object of the present invention to overcome the difficulties presented in a broadcast non-contact switching system when the two input power levels are unequal such that one cannot efficiently obtain appropriate output power levels.
Another object is to obtain efficiencies in excess of 99% with any input power ratio.
A further object is to enable the combining of three sources or transmitters by means of noncontact switching so as to achieve minimum losses.